# STR removes the possibility of a common present

1. Nov 7, 2014

### gamow99

In volume 74 of Analysis, pg 148-157 L Leininger writes:

I object that STR removes the possibility of a common present. For example, say that a beam of light flashes in all directions and we will call the time in which this happens, Jan 1, 3000. Traveler A is located 1 light year away from the flash and is traveling 95% the speed of light away from the flash. Given time dilation traveler A will learn about the event, let's say in year 3020, though I'm not aware of the exact calculations. Traveler A can reason that although they learned about the event in the year 3020, that given their speed and the speed of light, that the event occurred on Jan 1, 3000. Traveler B on Jan 1, 3000 was also located a light year away from the event but was traveling directly toward the flash. Let's say they learn about the event on Aug 1, 3000 though I'm not aware of the exact calculations. Now, although traveler A and B learn about the events at different times, they can both calculate their speed as a percentage of the speed of light and determine that the event happened at the same time for both of them, namely, Jan 1, 3000. Hence, there is a common present for both A and B.

2. Nov 7, 2014

### Staff: Mentor

Sure, they can use units where c=1, then speed will be calculated as a percentage of the speed of light. That is not the point. The point is which reference frame do they use when doing that calculations? Different reference frames will disagree on their speed, even in units where c=1.

3. Nov 7, 2014

### gamow99

Are you saying that although it is a fact for A that they are traveling at 95% c it is not a fact for B? Further, do you disagree that the flash of light occurred on Jan 1, 3000, although B learns about it on Aug 1, 3000 and A learns about it in 3020?

4. Nov 7, 2014

### Staff: Mentor

It is not a "fact" for either of them. There exists a reference frame where they are moving at .95 c. There also exists a reference frame where they are moving at .99999 c, and one where they are moving at .01 c, and yet another where they are at rest. All of these frames are equally valid and none can be considered to be more of a "fact" than any of the others.

There exists a reference frame where you, right now, are moving at .95 c.

5. Nov 7, 2014

### harrylin

I'm not sure if it is what you mean, but indeed one can choose one universal coordinate system in which one defines "universal time" including a common present (something like UT but a little better). As one can do that, it's wrong to claim that SR removes the possibility of a common present. Probably the author meant to write that standard inertial reference systems that are in relative motion can not agree about a common present for all locations - and he next elaborates why.

Last edited: Nov 7, 2014
6. Nov 7, 2014

### gamow99

Trust me, she believes that SR removes the possibility of a common present. I've met the author in person and she believes that, which is not shocking, many philosophers of time believe that there is no common present.

7. Nov 7, 2014

### Staff: Mentor

We can discuss the physics and the scientific theory here, but the philosophy is simply not part of PF.

Last edited by a moderator: May 7, 2017
8. Nov 7, 2014

### gamow99

Ok, I'm willing to believe this. However, is it possible for traveler A when they learn about the flash and they see what direction it is coming from and they see in what direction they are traveling to determine when the flash ocurred in the past? E.g., We can determine that the sun existed above the horizon 8 minutes ago.

9. Nov 7, 2014

### gamow99

We're discussing whether or not Leininger's beliefs are logically consistent or contradictory. We're not discussing philosophy.

10. Nov 7, 2014

### Staff: Mentor

Only if he assumes a specific reference frame in which to make the calculation. There is nothing which constrains the choice of reference frame, and the answer will change based on that choice.

Yes, because we assume a reference frame in which the sun is at rest. If we were to assume a different reference frame then we would get a different time.

11. Nov 7, 2014

### harrylin

We already agreed that such beliefs do not follow from the observed physics; and the link that Dalespam gave summarizes earlier discussions which explain that in detail.

12. Nov 7, 2014

### gamow99

What reference frame could you use to justify that the flash occurred in the year, say, 3010?

Could you give an example where we could calculate that the sun existed above the horizon 16 minutes ago?

13. Nov 7, 2014

### gamow99

Yes, but I would still like to see how it is rational to believe that the sun existed above the horizon 16 minutes ago. It seems that there are some frames of reference which would be contradictory to assume.

14. Nov 7, 2014

### Staff: Mentor

Let's use units of minutes and light-minutes (c=1). Let's consider the earth and sun to be at rest and set the earth at x=0 and set t=0 to be the moment when some light from the sun reaches the earth. Then in that frame the light was released from the sun at t = -8 and x = 8.

In the reference frame where the earth and sun are moving at -0.6 c the light that is released from the sun at t = -16 (and x = 16) and arrives at the earth at t = 0 (and x = 0).

15. Nov 7, 2014

### gamow99

Ok, that seems reasonable. Now how about this: It is possible for traveler A and traveler B to both assume that the light flash that I was talking about was at rest. Further even though they are two light years apart from each other they both agree that the time is 1/1/3000, although admittedly in real life it would be difficult to coordinate such an agreement on time. When A does his calculations and he assumes that the flash is at rest and that he is moving directly away from it at 95% c then he should conclude that the flash occurred at 1/1/3000. When B does his calculations and he assumes that the flash is at rest and he is traveling directly toward it at 95% c, then he should also conclude that the flash occurred at 1/1/3000.

16. Nov 7, 2014

### Staff: Mentor

They can assume that, but this assumption will conflict with more than a century's worth of experimental evidence showing that the speed of light is the same (and equal to c) for all inertial observers, as well as being completely inconsistent with the laws of electricity and magnetism, which have been around for even longer.

No matter how you position your inertial observers, and no matter how you time the release of the light signal, it will eventually catch up with and pass the observer, and when it passes that observer it will be travelling at speed c relative to him. There's no way of reconciling that behavior with the assumption that the flash is at rest.

17. Nov 7, 2014

### gamow99

I didn't mean that the photons were at rest but the thing that emitted the photons was at rest.

18. Nov 7, 2014

### Staff: Mentor

Then I'm not sure what point you're trying to make. The choice of which point I consider to be at rest is completely arbitrary (and I can choose from any point in the universe, not just the positions of the the two observers or the light source) and depending on which point I choose I will get different results for the "common present".

There is something that all observers everywhere will agree about, no matter what their speed and position and what which point they choose to be at rest: They will all agree that the flash of light was emitted before it was received by A and before it was received by B. But they will not agree about what A's clock reads "at the same time" that B receives the flash, nor what B's clock reads "at the same time" that A receives the flash.

19. Nov 7, 2014

### gamow99

Tell me if you agree with the following: Let's say a supernova occurs and at the moment of the supernova A and B are both one light year away from the supernova assuming the supernova is at rest. Let us also assume that when the supernova occurred by random coincidence they decided to synchronize their clocks to 1/1/3000 even though they were not communicating with each other. A is traveling directly away from the supernova at 95% c assuming the supernova is at rest, but B is traveling toward the supernova at 95% c assuming the supernova is at rest. When B learns of this event he will be able to deduce: "I was one light away from this supernova when it happened." When A learns of this event he will also be able to deduce: "I was one light away from this supernova when it happened." Even though B learns of this event before 1/1/3001 and A learns of it after 1/1/3001, both A and B will still agree that when they learned of the event, assuming the supernova is at rest, that they were located 1 light year away from the supernova when it happened.

20. Nov 7, 2014

### Staff: Mentor

This is the key assumption. If they both make this assumption then they are both using the same reference frame and will therefore agree on simultaneity. However, there is no reason that they need to make this particular assumption (use this reference frame), nor is there any reason that they need to make the same assumption as each other (use the same reference frame as each other). If they make different assumptions then they will get different results, as I showed above.